Normally a User Equipment (UE) in active mode in a cellular wireless communication network system is handed over from one (source) cell to the another (target) cell as it moves through the network, and data can be transmitted and received without significant interruptions due to these handovers.
The handover (HO) procedure can consist of many steps. In most cellular wireless communication systems, the handover is network controlled, i.e. the UE is commanded by the network when to connect to another cell. The handover is prepared, i.e. the target cell (the cell that UE is moving to) is prepared. Further, the UE is assisted, i.e. the UE provides measurement reports before handover to the serving cell to assist the decision to do handover preparation of target cell(s), and when to leave the serving cell/connect to the target cell.
In the context of handover, the serving cell before handover is often referred to as the source cell. After successful handover the target cell becomes the new serving cell, and so on.
To assist mobility control decisions, the UE can measure several cells and report the results to the network. Different networks and network deployments can have different detailed behaviour, but in most networks it is natural to trigger handover when signal reception from target cell is better than from source cell.
When the serving eNB, associated with the serving cell, receives a measurement report and if it desires to handover the UE to another cell it performs a handover preparation to that cell. Handover preparation involves a signalling exchange between one eNB and another eNB. The source cell requests the handover and passes over UE context information; the target cell decides if it can admit the UE and either accepts or rejects the handover.
Following a successful preparation, the handover execution takes place. The source cell issues the Handover Command to the UE. If, and when the UE receives this correctly it synchronises to the new target cell and sends a synchronisation message on the Random Access Channel (RACH). The target cell then issues an allocation to the UE so that the UE can send a Handover Confirmation message to the target cell.
The final steps, the Handover Completion, do not involve the UE. The source eNB is able to forward data (unacknowledged downlink packets) to the target eNB, and the S1-U interface from the S-GW must be switched from the source to the target (“path switch”). Finally, if the handover is successful, the target eNB issues a UE Context Release message to the source eNB.
In 3GPP there has been considerable study into Self-Organising Networks (SON) for LTE. One part of this is the Handover Parameter Optimisation a.k.a. Mobility Robustness Optimisation (MRO) which is aiming at optimising mobility parameters. This functionality is supported by letting the UE indicate the last serving cell when re-establishing the connection after a failure has occurred.
The reporting from the UE is slightly different depending on the state of the UE. During a Radio Link Failure (RLF), the UE first tries to re-establish the Radio Resource Control (RRC) connection and if this is not successful, it will return to idle mode and be triggered by higher layers to establish a new RRC connection to a cell.
When re-establishing the RRC connection (using the RRC Re-establishment procedure), the UE will provide the identity of the last serving cell (i.e., Physical Cell Identity, PCI) and the UE identity used in the last serving cell, i.e., Cell Radio Network Temporary Identifier (CRNTI), as well as (optionally) a 16 bit long field called shortMAC-I. In addition to this, the UE may also indicate that it can provide a RLF report, which for example contains a set of radio measurements at the point of failure. The eNB receiving the re-establishment request may choose to reject this, and in this case, the RLF report will not be transmitted.
When establishing an RRC connection from idle state, the UE will not include the PCI, CRNTI or shortMAC-I in the request itself. Instead, the UE may separately send the RLF report which includes information about for example last serving cell as well as the radio measurements at the point of failure.
MRO is defined to distinguish between three different failure cases, namely:
Too late—a scenario where handover is triggered too late, for example when a failure occurs in a source cell before or after handover is initiated and where the UE re-establishes the connection in another cell.
Too early—a scenario where handover is triggered too early, for example when the UE fails to connect to the target cell or when the UE connects but the radio link fails shortly after the (successful) handover.
Wrong cell—a scenario where UE is told to connect to one cell but fails there during or after handover and the UE re-establishes the connection in a third cell.
The general idea is that the cell where the UE attempts to re-establish will send an RLF Indication message over the X2 interface to the last serving cell and this cell can use the information to adjust the mobility parameters. In some cases, the information is forwarded further by the last serving cell to a third cell. This is because during the handover procedure, after the RACH succeeds, the UE will consider the target cell as the serving cell. Hence, in the two latter scenarios above, when the failure occurs shortly after a successful handover, the information must be forwarded to the serving cell before handover and not the last serving cell. This is accomplished with the Handover Report message.
Example of a too early handover (see FIG. 1). Handover from cell A (part of eNB A) to cell B (part of eNB B) succeeds. Shortly afterwards a RLF occurs, UE attempts re-establishment in cell A (1), eNB A sends a RLF Indication to eNB B (2), cell B recognises that the failure occurred shortly after a handover (by using a timer) and eNB B sends a Handover Report to eNB A (3). In the future, cell A can avoid similar failures by making it more difficult to handover to cell B which represents an unreliable target cell, e.g., only an isolated pocket of good coverage.
Example of a handover to wrong cell (see FIG. 2). Handover from cell A to cell B succeeds. Shortly afterwards a RLF occurs, UE attempts re-establishment in a third cell C that is part of eNB C (1), eNB C sends a RLF Indication to eNB B (2), eNB B recognises that the failure occurred shortly after a handover (it uses a timer) and sends a Handover Report to eNB A (3). In the future, call A can avoid similar failures by making it more difficult to handover to cell B which represents an unreliable target cell, e.g., only an isolated pocket of good coverage, or it can make it easier to handover to cell C which represents a better target cell.
However, to improve handover performance in such systems there is a need for a method by which mobile stations can be identified.